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Project Motivation

Diversion of food wastes from municipal solid waste (MSW) landfills is being practiced and promoted in several states (e.g., California, Oregon) and at several commercial institutions/restaurants (e.g., Costco), and is becoming prevalent on many college campuses (e.g., Carnegie Mellon, Duke, University of South Carolina). As this requirement (or desire) for diversion of food waste becomes more prevalent, there is a need to ensure that the most appropriate technology be utilized. For this to occur, the evaluation of newer technologies, such as hydrothermal carbonization, is necessary. Hydrothermal carbonization (HTC) is a thermal conversion technique that converts waste materials to a valuable, energy-rich, coal-like material (often termed hydrochar, biochar, or char).

Hydrothermal Carbonization

Hydrothermal treatment is an attractive waste management strategy for wet organic wastes. During hydrothermal conversion, wet feedstocks are heated in subcritical water to between 180 to 350 oC under autogenous pressures. The feedstock is decomposed by reaction mechanisms similar to those in dry pyrolysis. A high energy, high carbon- containing solid residue that has characteristics similar to coal is formed, as is a liquid and gas product, as illustrated in Figure ES-1.  There is potential to gain energy from each of these products. Unlike other thermochemical conversion processes (e.g., incineration, pyrolysis), one of the most important components of this process is liquid. Heated liquid has been shown to have a catalytic effect on feedstock carbonization, which is attributed to the significant change in liquid behavior at high temperatures.

There are many potential advantages associated with using HTC as a solid waste treatment tool. Because the majority of the carbon remains integrated within the liquid and/or solid material (depends on reaction temperature/time), such conversion has the potential to substantially reduce fugitive greenhouse gas emissions from current management/treatment processes. Several studies report that the majority of carbon initially present remains integrated within the hydrochar material (50 – 90% of initially present carbon). In addition, the recovered solids may be used in applications such as environmental remediation and soil augmentation.

Feedstock initial moisture and carbon contents are critical during carbonization. The input energy required for hydrothermal processes is lower than incineration when feedstock moisture content is > 8% (by wt.) and lower than pyrolysis when feedstock moisture content is greater than 25% (by wt.). These results suggest guidelines for conditions that may be more ideally suited for carbonization. It is important to note, however, that efficiencies of carbonization must also be considered before a final determination can be made. Feedstock carbon content also plays an important role on carbonization. As the carbon content of the initial feedstock increases, the carbon content, and thus energy content, of the recovered solids also increases. Conversion of food wastes to an energy source is extremely advantageous, as renewable energy is something in which there will always be a market.